Analysis of Optical Cross Sections

In this section, we shall review various factors that must be considered in accurate evaluation of center parameters from the raw data. First, the extent of the (already mentioned) lattice relaxation must be considered (see below). Second, there can be additional complications, such as excited states or a held dependence of the cross section. In any case, one tries to separate out such complications and thus obtain an electronic cross section. This latter can then be compared to appropriate theory (Section 12). 

A probable role of an excited metastable state for the oxygen center of GaAs has already been mentioned in connection with our discussion of the configuration coordinate model (Section 10b). This was suggested by Vincent and Bois (1978) to explain a slow decrease of photocapacitance observed after 

Fig 18. Comparision between the spectral dependence of the photoneutralization cross section ( j°i) for in GaP without field (dotted line, taken in bulk by photoluminescence excitation) and with field (dashed line, taken in a junction by photocapacitance). [After Monemar and Samuel-son (1978, Fig. 3), with the photocapacitance data provided by . H. Henry, and data taken at 190°K.] 

The next important aspect to be considered is the electron-phonon interaction (lattice relaxation). Here, the effect of momentum conserving phonons, or promoting modes, can in principle be included in the electronic cross section this is discussed, for instance, by Monemar and Samuelson (1976) and Stoneham (1977). However, the configuration coordinate (CC) phonons (or accepting modes) are treated separately. The effect of these CC modes is usually expressed by the Franck-Condon factor dF c, where this factor is the same as the defined in our Fig. 16. Thus assuming a single mode, 

The electronic optical cross section a(hv) can be expressed in the dipole approximation as 

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Recombination at and excitation from deep levels are emphasized. Nonradiative transitions at defect levels—Auger, cascade capture, and multiphonon emission processes—are discussed in detail. Factors to be considered in the analysis of optical cross sections which can give information about the parity of the impurity wave function and thus about the symmetry of a particular center are reviewed. 

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